SYENITE OCCURRING IN PORTUGAL. AB 
angle of about 34°. The two mountains are united at their base, 
thus forming a valley which is called the “ Barrocal,” and which, 
owing alike to its beautiful climate and exuberant vegetation, is 
Yooked upon as an El Dorado by the inhabitants. 
The local nature and position of the crystalline mass both seem 
to differ greatly at different points. The rock often changes abruptly 
from coarse to fine-grained ; and all varieties, from the coarse crystal- 
line to a porphyritic, from the unaltered to a metamorphic variety, 
are represented. A very fine-grained, almost compact variety 
intersects the coarse crystalline mass in narrow veins, and occurs 
likewise interbedded as dykes. The surface of both mountains is 
strewed with large blocks which often form conically-shaped mounds 
and are frequently weathered. On the whole, it would appear, from 
the specimens ! have examined, that the main portions of the Foya 
and the Picota, as well as the eastern slopes, consist chiefly of the - 
fine-grained variety ; whilst on the southern slopes the rock is more 
coarse-grained. Again, the greywacke strata at the eastern ex- 
tremity, near Marmeleta, are altered and decomposed at the point 
of contact ; whereas to the south, as near Coldas, no alteration is 
perceptible. Not the least interesting feature in this district is the 
large number of veins of Tertiary eruptive phonolitic and basaltic 
rock which occur throughout the crystalline mass, and coincide with 
similar veins in the Jurassic and Tertiary strata near Cape St. 
Vincent. These veins intersect the surface in all directions, and 
warrant the conclusion that the whole district has lost much of its 
former height, an enormous mass of rock having probably been 
removed by denudation. 
An examination of Foyaite with the naked eye shows its essential 
constituents to be monoclinic felspar (orthoclase), eleeolite (the 
coarse variety of nepheline), and hornblende. 
Orthoclase is largely predominant, generally of a greyish-white 
colour, showing strong greasy lustre on the planes of cleavage. 
Elzolite shows hexagonal or rectangular contours, and is of a 
reddish-grey or flesh colour with strong greasy lustre. Hornblende 
occurs chiefly in long, slender, prismatic crystals of a greenish-black 
colour, with strong lustre on the planes of cleavage. The porphyritic 
variety shows large crystals of orthoclase, and in places also crystals 
of elzolite imbedded. The accessory constituents, determinable 
with the lens, are brownish-yellow titanite, dark lamellar biotite, 
grains of magnetite, and traces of pyrite. 
But under the microscope Foyaite exhibits a far more varied and 
interesting appearance. Not only do all the constituents I have 
mentioned present themselves with all their peculiarities of structure, 
of intergrowth, of juxtaposition, but nosean and sodalite are found 
to be present as characteristic accessories under altogether novel 
conditions, whilst triclinic felspar, muscovite, haematite, and apatite 
are also met with. 
Orthoclase is predominant in large crystals, not always with regular 
outlines, of a greyish-white colour, dull, generally translucent, 
showing numerous Carlsbad twins in polarized light. It is nearly 
